Layered sodium metal oxides have attracted considerable interest as cathodes for Na-ion batteries partly due to the fact that seven layered O3-NaTMO2 can be synthesized by simple solid state reactions and show the capability to intercalate Na ions reversibly, where TM is a single oxidized 3d transition metal ion selected from Ti, V, Cr, Mn, Fe, Co, Ni and O3 denotes the oxygen stacking. This is very different from their Li analogues, where only LiCoO2 and LiNiO2 reversibly intercalate Li ions.